Optical fibers are commonly used today for the transmission of signals of all sorts, including communication and data signals. Optical fibers can be single mode fibers (typically employed in long-distance communication), which have only one strong propagation mode, or multi-mode fibers, in which light transmitted in the different modes arrives at different times, resulting in dispersion of the transmitted signal.
Single mode fibers transmit signals between transceivers (ie., devices that can both transmit and receive optical signals) via pairs of fibers. More specifically, one fiber of the pair will transmit signals from the first transceiver to the second, and the other fiber of the pair will transmit signals from the second transceiver to the first. In this manner, optical signals are not traveling along the same fiber in different directions, as such activity could interfere with both signals.
This pairing arrangement would be fairly simple to organize for two transceiver devices that are permanently optically connected, but in practice transceivers are typically connected through a much larger network of optical fibers, connectors and patch panels. For example, a common optical system includes multiple transceivers at one end, patch cord pairs that are connected to the transceivers and to a duplex adapter mounted on a patch panel, a fan-out unit connected to the duplex adapter that connects to a multi-fiber ribbon cable (12 fibers per ribbon is common) via an array adapter, a second fan-out unit connected to the opposite end of the ribbon cable via a second array adapter, and corresponding transceivers connected via patch cord pairs to the second fan-out unit through another duplex adapter. Thus, clearly it is important to be able to track individual optical fibers in the various devices and cables between the transceivers in order to ensure that the individual transceivers are connected as desired.
To ensure intermateability of cabling components and signal polarity, standards have been created to define arrangements of fibers, cables, adapters and connectors. For example, one such standard for array connectors, TIA-604-5B, is directed to MPO fiber optic connector intermateability. Another standard, TIA 568-B.3 with proposed addendum written by committee TR-42.8, is directed to maintaining optical fiber polarity with systems using array connectors and adapters, including MPO's. This proposed addendum discusses four different methods of creating an optical path from the transmit side of one transceiver to the receive side of another transceiver. One method, termed “Method A,” is intended to “link multiple duplex optical transceiver ports or to link two parallel optics transceiver ports . . . ” Systems built using Method A utilize Type A ribbon cables, Type A adapters, Type A transitions and 568B.3 patch cords.
One of the characteristics of a Method A optical path is that the array adapters are “key up to key up,” or “aligned-key” style adapters. This term refers to the orientation of small projections, or “keys,” located on the terminating bodies of cables that enable one connecting the cables to orient them correctly relative to the adapter (an incorrectly oriented cable would align the wrong fibers, which would prevent proper transmission of optical signals). Aligned-key adapters are less traditional for array connectors than the standard “key up to key down, or “opposed-key” adapters, but are an acceptable alternative defined as “key option k=2” in TIA-604-5B. To aid cable termination during manufacturing, some array connectors (including MPO's as an example) include a “body mark” (any visual indicia, often a white paint mark) on the body portion of each terminal assembly that indicates how the cable should be oriented for connection. Conventionally, the body marks of an array connector are located on the same side of the cable as a fiber designated “Fiber 1” and are to be on the left side of the body portion when viewed facing the exposed ends of the optical fibers with the key projecting upwardly.
One of the difficulties presented by “aligned-key” connections in Method A systems is the actual contact angle between mating fibers. Most cables terminate with a ferrule that exposes the ends of the fibers of the cable for optical interconnection with another cable. Once the fibers of a cable are inserted into a ferrule and bonded thereto, the exposed ends of the fibers are polished to improve the transmission of signals between joined fibers. The polishing can either be performed normal to the axes of the fibers (known as “flat” polishing), as is typically done for multi-mode applications, or at a slight oblique angle to the axes of the fibers (known as “angle” polishing), as is typically done for single mode applications. Angle polishing is typically preferred for single mode applications, as it reduces the risk that inadvertent light reflection from the end of the fiber will occur during transmission. However, angle polishing of ferrules requires that mating ferrules be oppositely angled; i.e., the angled face of one of the mating ferrules must face slightly upwardly and the angled face of the other ferrule must face slightly downwardly in order for these faces to abut correctly for light transmission. Each ferrule of an array connector will typically include a ferrule mark (again, typically some visual indicia such as a paint mark or molded-in designation) that indicates to one terminating the cable how to orient the ribbon of fibers in the ferrule prior to bonding and polishing. Conventionally, the ferrule mark is located on the same side of the cable as the aforementioned Fiber 1 and is also on the same side of the cable as the connector body mark. This placement instructs the installer to polish the contact surface such that, when the ferrule is viewed from the side of the ferrule that includes the ferrule mark with the exposed fibers facing to the right, the lower edge of the ferrule is worn away during the polishing process. In this conventional termination, the key on the connector body is facing up.
Because of the requirements associated with Method A mentioned above, with conventional components it is not possible to follow the Method A connectivity arrangement while still having both (a) the conventional “aligned-key” mating with adapters and (b) angle polished surfaces for the ferrules of the cables (in fact, the addendum to TIA 568-B.3 proposed by TIA TR-42.8 states that “all connectors used in Connectivity Method A must be flat polished; angle polished connectors cannot be connected key-up to key-up”). As such, an optical system that conforms to the connectivity requirements of Method A and meets these other configuration requirements would be desirable for single mode performance and polarity assurance.